136 



FERTILIZATION OF THE Ol'L'M 



Allolobophora (Foot), in the butterfly Pieris (Henking), and in the 

 gasteropod Physa (Kostanecki and Wierzejski). The agreement 

 between forms so diverse is very strong evidence that this must be 

 iv-Mrded as the typical derivation of the centrosome. 1 



The facts may be illustrated by a brief description of the phe- 



Fig. 67. Maturation and fertilization of the egg of the mouse. [SOUOTTA.] 

 A. The ovarian egg still surrounded by the follicle-cells and the membrane (s.p., zona pel- 

 lucida) ; the polar spindle formed. B. Egg immediately after entrance of the spermatozoon 

 (sperm-nucleus at o*). C. The two germ-nuclei (tf, ) still unequal; polar bodies above. 

 D. Germ-nuclei approaching, of equal size. E. The chromosomes forming. F. The minute 

 cleavage-spindle in the centre ; on either side the paternal and maternal groups of chromosomes. 



nomena in the sea-urchin Toxopneustes (Fig. 69). As described at 

 p. 146, the tail is in this case left outside, and only the head and 

 middle-piece enter the egg. Within a few minutes after its entrance, 

 and while still very near the periphery, the lance-shaped sperm-head, 

 carrying the middle-piece at its base, rotates through nearly or quite 



1 Cf. P . 156. 



